Control valves used in freight car brake systems are well known. If such control valves supply air pressure to the brake cylinder of a freight car, and the plumbing between the control valve and the freight car has a leak, then the brake cylinder will not maintain the original set pressure. This could be due to, for example, a leaking packing cup in the brake cylinder, a leaking hose, or any other reason that would lead to the leaking of pressure between the connection of the control valve and the brake cylinder.
Various approaches to resolving this issue are provided in, for example, U.S. Pat. Nos. 4,474,412; 4,493,511; and U.S. Pat. Pub. No. 2015/0061360, all of which are incorporated by reference herein in their entireties. In U.S. Pat. No. 4,474,412, a brake cylinder pressure maintaining valve is connected to a brake pipe, brake cylinder pipe and volume reservoir, as well as to an emergency reservoir. The brake cylinder pressure maintaining valve operates to provide air from the volume reservoir to the brake cylinder pipe and thus the brake cylinders when pressure in the brake cylinder pipe drops below a predetermined level, thus ensuring a minimum brake application pressure, regardless of the piston travel of the brake cylinders.
U.S. Pat. No. 4,493,511 discloses a brake cylinder pressure maintaining valve which, during brake application, takes air from the brake pipe and feeds it into the brake cylinder until it reaches minimum brake pressure. To avoid the need for an extra valve, the brake cylinder pressure maintaining valve is used as a quick service bulb exhaust valve during brake release and means are provided to ensure that the brake cylinder pressure maintaining valve opens immediately on brake release. Once the brakes have been applied, pressure in the brake cylinder is maintained from the quick service bulb. Brake cylinder exhaust pressure taken from a point after the brake cylinder exhaust valve, but before the brake cylinder exhaust choke, is directed to a chamber above the inshot and brake cylinder maintaining valve diaphragm. Since the pressure in a port upstream of the brake cylinder exhaust choke attains a pressure within about 10 psi of the chamber, a spring in another chamber can open the brake cylinder maintaining valve. Thus, immediate opening of brake cylinder maintaining valve occurs when the bi-stable operator moves to the release position. Bulb pressure from a QS bulb is then able to flow via the BC maintaining choke and fall with the fall of brake cylinder pressure.
U.S. Pat. Pub. No. 2015/0061360 discloses a brake cylinder pressure maintaining valve which utilizes an input and output diaphragm as a differential relay for maintaining brake cylinder pressure. The valve utilizes ports connected to an emergency reservoir, auxiliary reservoir and the brake cylinder, and is controlled by a balance of these pressures.
Such prior art approaches to brake cylinder pressure maintenance have disadvantages. For example, the approaches discussed in U.S. Pat. Nos. 4,474,412 and 4,493,511 are relatively complicated and difficult to implement. The approach discussed in U.S. Pat. Pub. No. 2015/0061360 requires the use of both emergency reservoir and auxiliary reservoir pressures, which could potentially be undesirable. Accordingly, improved valves for maintaining brake cylinder pressure are desired.